1. Field of the Invention
The present invention relates to electrical communications systems and, in particular, to a frequency shift keying (FSK) modulator.
2. Discussion of the Prior Art
The basic function of a communications system is to transmit information from a source to a destination as fast and as accurately as possible. The source and the destination are physically separated from one another and are connected by a communications channel.
There are two types of information sources: analog information sources and discrete information sources. Analog sources, such as a microphone, produce a continuous signal. Discrete sources, such as a digital computer, generate a signal consisting of a sequence of pulses. Analog signals can be converted to discrete signals by utilizing sampling and quantizing techniques.
Communications channels which are designed to handle voice transmissions (i.e., the telephone network) have characteristics which make it difficult for them to transmit digital signals. To permit the transmission of digital bit streams over a voice channel, it is necessary to utilize the digital data pulses to modulate a carrier waveform having a frequency which is compatible with the voice channel.
The equipment which performs the required modulation is generally referred to as a "modem". The term "modem" is an acronym for Modulator-DEModulator, since the equipment typically includes the capability not only to modulate transmitted signals but also to demodulate received signals.
Generally speaking, as stated above, a modulator receives a serial digital data bit stream from an information source and converts the bit stream to a waveform suitable for transmission over the communication channel. In addition to matching the frequency spectrum of the transmitted signal with the characteristics of the communication channel, the modulator also minimizes the effect of signal distortion caused by the non-ideal nature of the communications channel.
There are three basic modulation types: (1) amplitude-shift keying (ASK), (2) frequency-shift keying (FSK), and (3) phase-shift keying (PSK).
Data transmission systems which operate at lower data rates, i.e. 1200 baud or less, typically utilize FSK modulation. In FSK modulation, the two binary states are represented by two different frequencies and are detected by using two frequency tuned sections, one tuned to each of the two bit frequencies. The demodulated signals are then integrated over the duration of a bit and a binary decision is based on the result.
The FSK modulator section of the circuit of the present invention utilizes a spectrum controller which, during each data transition, sends out six intermediate frequencies to smooth the frequency changeover.
Thus, the present invention provides a frequency phase shift keying modulator which generates a strobing signal in response to transitions in the serial digital data input signal. A digital rate generator responds to the strobing signal by providing an output representative of a destination frequency. The carrier waveform is then generated utilizing the output of the rate generator to provide the frequency modulated output signal.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized.